This invention is directed to heat exchanger fin collars, and more particularly, to an improved method for manufacturing the fin collars to have an extended tube-contact portion, for improved heat exchange efficiency and better galvanic corrosion durability.
Plate-fin coil air-side surfaces are formed in progressive dies. There are several variants of these dies which include draw forming, drawless forming, fin-per stroke, and high collar dies. For each method, a primary consideration is the formation of the tube contact cylinder of the fin collar, which is used as the contact area between the fin collar and the heat exchanger tube. From both thermal performance and corrosion durability perspectives, a greater contact area is advantageous. Also, for many applications a high fin density is desirable. Therefore, it is preferable to have a large number of fin collars with a relatively small size contact leg, but with a large percentage of the contact leg in contact with the heat exchanger tube. Also, the manufacturing process should be flexible in making fin sizes for a wide range of fins per inch and capable of producing a good and repeatable collar geometry. Current methods fail to adequately achieve these goals. As represented in FIGS. 4 and 4a, most fin collars formed in accordance with prior art methods have tube-contact legs which only contact the tube surface over a very short distance, essentially at the apex of the contact leg""s radius.
For a coil made with bare finstock, a relatively small contact area between fin and tube will provide thermal transport with minimal thermal resistance. However, if the finstock has an organic film or other coating with a significant thermal resistance, a larger contact area provides substantially improved performance.
With current practices, while the length of the contact leg is somewhat adjustable or flexible, based on the ability to perform multiple drawing stages, the resulting contact leg is frequently not formed sufficiently straight. The limitations of various current fin forming methods can be seen by referring to FIG. 5. The fin collar formed from this method includes contact legs that are curved and do not effectively cover the surface of the heat exchanger tube, as shown in FIG. 4a, thereby inefficiently contacting the tube surface and accordingly, failing to achieve the best heat exchange relationship therewith.
More specifically, in the draw forming method of FIG. 5a, a sheet or strip of fin stock material is formed with a button therein. The height or depth of the button may be increased or decreased to adjust the fin density and the length of the fin collar contact leg. Accordingly, a number of drawing stages are used to shape the contact leg of the fin collar. The button is then pierced and the fin collar is shaped, straightened and flared for forming the desired contact leg. Corrosion durability of an aluminum fin/copper tube heat exchanger is inversely proportional to the exposed area of the copper tube in the fin pack of the coil. This is because the primary corrosion mechanism for these heat exchangers is galvanic corrosion. Reducing the cathodic copper area proportionally decreases the corrosion current. In addition, improving the straightness of the collar contact area decreases access of electrolyte to the copper/aluminum contact area of the galvanic couple. More complete coverage of the tubes by the aluminum collar improves corrosion durability. The amount of electrolyte that can be stored in the collar crevice is also a function of the collar design. The reduction in the electrolyte content proportionately reduces the galvanic current.
The drawless forming method of FIG. 5b begins with a piercing and burling step and thereby lacks the multiple drawing stages of the draw forming method and, accordingly, lacks the flexibility of adjusting the contact leg length. In the first step, fin stock is pierced and burled to form a pre-contact leg. The pre-contact leg is ironed for straightening and limited lengthening and finally, the tip of the leg is flared or curled. Accordingly, this method lacks the flexibility of adjusting the contact leg length. Similarly, the single shot method shown in FIG. 5c also lacks flexibility, starting with a piercing step, then a burling step to bend and form the pre-contact legs, and finally a flaring step for flaring or curling the ends of the contact legs. The high fin method of FIG. 5d has substantially the same steps as the draw forming method with additional ironing steps between the piercing and burling and flaring steps so as to somewhat improve the straightness of the contact leg. However, the high fin method suffers from the same defects or shortcomings as the draw forming method, described above.
There exists a need, therefore, for an improved fin collar and forming method whereby the fin collar is formed with a substantially straight contact leg and greater contact area and whereby the method has the flexibility to provide for any desired length of the contact leg while maintaining its straightness as well as good physical and material characteristics.
The primary object of this invention is to provide an improved method for manufacturing heat exchanger fin collars and an improved fin collar design.
Another object of this invention is to provide an improved heat exchanger fin collar which has a substantially straight contact leg and greater contact area between the fin collar and the tube, for a high level of heat exchanger tube contact.
Another object of this invention is to provide an improved method for manufacturing a heat exchanger which provides for more complete coverage of the copper tubes and thus yields heat exchangers with improved corrosion durability.
Still another object of this invention is to provide an improved method for manufacturing heat exchanger fin collars, wherein the method allows for flexibility in the length of the fin collar and a greater tube-contact leg to achieve greater contact area between the fin collar and the tube.
Yet another object of this invention is to provide a method for forming heat exchangers which reduce the amount of potential electrolyte volume between the fin collar and the tube-contact leg.
The foregoing objects and following advantages are achieved in part by the heat exchanger fin collar of the present invention, for plate-fin collar style heat exchanger having close tolerance dimensions for achieving greater contact area on the tube. The fin comprises an elongated fin portion for dissipating heat and a leg connected with the fin portion. The leg has a height and includes a straight contact portion substantially perpendicular to the fin portion, wherein the contact portion has a contact height along which the contact portion contacts the tube. The contact height is in the range of 0.008 to 0.080 inches for a fin density range of 25 to 10 fpi. It also includes a first curved end portion having a first radius extending from a first end of the contact portion and a stepped transitional portion connecting the contact portion and the elongated fin portion. The transitional portion has a second curved end portion having a second radius, wherein the second curved end portion extends from the contact portion opposite the first end.